Strangely, you did not include barometric pressure, which can vary wildly over the course of the day. Looking at min/max in my area, it's as much as 5.17%, which correlates to air density directly.
The reason I did not mention barometric pressure in my post, is that I could not conclude from Andyw2100’s data a reliable trend to determine the difference of air pressure in July/August vs. June/July. Assuming highher humidity and higher temperatures in July/August vs. June/July, on another hand, is a safe bet.
SAE also excludes humidity from STP/NTP.
Somewhat OT here, but
SAE J1349 does require tests to be conducted at a specific RH of 50%, it is just that these tests have sufficient accuracy for a relatively wide range of values of relative humidity: +/-20%.
9.1.6 AMBIENT CONDITIONS
The test must be conducted at the following ambient conditions:
a. Air Temperature: 25 °C ± 10 °C
b. Barometric Pressure: 90-105 kPa
c. Humidity: 50 % R.H, +/-20.
d. Wind speed: less than 7 m/s (15 mph)
e. Road Surface: A closed course, with dry, flat, level hard-paved surface.
Somehow this seems to work for rating everyday things, include microprocessor controlled forced induction control, which needs realtime measurement of temperature and pressure. That also means whatever the effect of humidity is, it requires no active compensation.
Yes, the effect of air humidity on air density and, therefore drag, is not very intuitive, and people usually dismiss it without taking enough time to understand why.
As I mentioned in my original post I’ve found this empirically, trying to understand why my energy consumption was different for my morning commute, given near equal atmospheric conditions otherwise. So here is what I found after doing some digging.
According to the
Avogadro Law equal volume of all gasses, at the same temperature and pressure, have the same number of molecules. Base on this, a gas with the higher molecular weight has proportionately higher density. Since molecular weight of dry air is 28.966, while molecular weight of the water vapor is 18, the water vapor is almost 40% lighter than dry air. So each molecule of water that is replacing other gasses in dry air
reduces total density of the air mixture.
As an example, using
this calculator, the relative humidity at 75°F air, with dew point of 50°F is 41.44%, while dew point of 70°F at the same air temperature yields relative humidity of 84.47%. Converting 41.44% RH to absolute humidity using
this calculator produces absolute humidity of 9%. Relative humidity of 84.47% at 75°F is equivalent to an absolute humidity of 18.35%.
Finally, calculating molecular weight of the air mixture at 75°F with absolute humidity of 18.35% vs. 9.0% indicates that air mixture containing 18.35% of water vapor is 3.68% less dense than air mixture containing 9% of water vapor.